Problems With Excessive Entrance Velocity in Water Wells

Well Design

• Wells are deep shafts that penetrate layers of earth's surface. The shafts are dug or drilled in areas above aquifers and penetrate to or near the aquifer while picking up moisture from layered rock and stone along the way. Wells can be as narrow as 2 inches in diameter or as wide as several feet. They can be anywhere from a few to hundreds of feet deep and can pump as little as 50 gallons of water per hour to as much as 4,000 gallons of water per hour. Despite these differences, wells have the same purpose and similar design. Wells are meant to draw water from below ground to the ground's surface and feature a shaft, a bottom, a pump and well walls.

Entrance Velocity

• Water enters a well through its porous walls at a rate measured in feet per second. The recommended entrance velocity of water into a well is within a range of 0.1 feet to 1.5 feet per second. Higher entrance rates cause lower well efficiency rates. The greater the amount of water in the well, the greater a pump's workload. This greater workload requires greater electricity use and increases the likelihood that the pump will fail. The entrance velocity is affected by environmental factors as well as by the type of casing and screens placed within the well.

Well Screens

• Water quality is greatly affected by the amount of detritus present in the water supply. Some wells are drilled through sandy soils, others through rock and others through layers of both rock and sand. Water naturally pushes sediments into the well, making it necessary for well-builders to develop obstacles and screens to prevent sediment accumulation. Well screens are placed against well walls and can be made of wire mesh, slotted PVC tubes or louvered screens. On the opposite side of the screens, between the screen and well wall, is an area packed with gravel or coarse sand. The gravel pack and the screens serve to filter sediment from the water, but they also slow the entrance velocity. Water flows from the aquifer into the gravel pack, then through the screens and into the well.

Considerations

• When water enters the well, gravity pulls it toward the well's bottom. Sediment settles into the sump area, while a pump located just above the sump area pushes water to the surface. High entrance velocities can overwhelm the pump with both water and sediment, resulting in water contamination and well inefficiency.